In the context of global policy-driven innovation and industrial chain collaboration, the new energy vehicle sector, particularly electric cars, has entered a phase of accelerated market penetration and competitive restructuring. As a key player in this domain, China EV manufacturers are focusing on enhancing driving performance and energy efficiency. For dual-motor four-wheel-drive battery electric vehicles, integrating a disconnect device between the front axle electric drive assembly and the differential enables timely switching between four-wheel-drive (4WD) and two-wheel-drive (2WD) modes. This configuration addresses the need for improved drivability during mode transitions while optimizing energy consumption. When power demand or handling stability is required, the vehicle switches to 4WD mode; otherwise, it operates in 2WD mode to eliminate front axle drag losses, allowing the front motor to shut down and the rear motor to operate in high-efficiency regions. This not only enhances the overall efficiency of electric cars but also extends their driving range, a critical factor for China EV adoption. However, the engagement and disengagement of the disconnect device can impact driving comfort and performance. This study explores precise control strategies for these transitions and evaluates drivability through objective metrics, aiming to boost the competitiveness of electric cars in the global market.
The vehicle configuration analyzed in this research involves a dual-motor four-wheel-drive electric car equipped with a disconnect device. As illustrated in the figure below, the system includes a front motor, rear motor, inverters, a battery management system (BMS), and a disconnect device controller, all coordinated by a vehicle control unit (VCU). This setup allows for flexible energy flow management, crucial for optimizing the performance of electric cars. The disconnect device, positioned between the front motor and the differential, enables seamless mode shifts, which are essential for maintaining drivability in various driving scenarios common in China EV applications.
In 2WD mode, the disconnect device is disengaged, and the front motor is shut down and stationary. The vehicle is solely driven by the rear motor, with energy flowing from the battery through the rear inverter to the rear motor. This mode minimizes energy losses by eliminating drag from the front drivetrain, making it ideal for steady-state driving conditions in electric cars. The energy flow can be represented as:
$$
E_{2WD} = P_{battery} \times \eta_{rear} – P_{loss,rear}
$$
where \(E_{2WD}\) is the effective energy in 2WD mode, \(P_{battery}\) is the battery power output, \(\eta_{rear}\) is the efficiency of the rear motor and inverter, and \(P_{loss,rear}\) accounts for rear drivetrain losses. This configuration significantly improves the range of electric cars, especially in urban environments where China EV models are prevalent.
In 4WD mode, the disconnect device is engaged, and both front and rear motors contribute to driving. The VCU distributes the driver’s torque demand between the axles based on optimal efficiency and stability criteria. The energy flow involves parallel paths:
$$
E_{4WD} = (P_{battery,front} \times \eta_{front} + P_{battery,rear} \times \eta_{rear}) – (P_{loss,front} + P_{loss,rear})
$$
Here, \(E_{4WD}\) is the effective energy in 4WD mode, \(P_{battery,front}\) and \(P_{battery,rear}\) are the power allocations to the front and rear motors, respectively, and \(\eta_{front}\) and \(\eta_{rear}\) are their respective efficiencies. This mode enhances traction and acceleration, which is vital for electric cars operating in diverse road conditions, a key focus for China EV development.
The transition between drive modes is governed by specific conditions and a phased control strategy to ensure smooth operation. For switching from 2WD to 4WD, conditions include rapid acceleration requests (e.g., accelerator pedal opening exceeding 70%), activation of stability systems like TCS or VDC, or fault conditions in the rear motor. Conversely, switching from 4WD to 2WD occurs under gentle acceleration (e.g., pedal opening below 60%), no stability system activation, or front motor faults. These criteria are designed to balance performance and efficiency in electric cars, aligning with the goals of China EV manufacturers to deliver responsive and economical vehicles.
The mode transition control involves a stepwise torque transfer process. For 2WD to 4WD shifts, it begins with front motor speed synchronization, where the front motor adjusts its speed to match the wheel speed, minimizing the rotational difference:
$$
\Delta \omega = |\omega_{input} – \omega_{output}| < \epsilon
$$
where \(\Delta \omega\) is the speed difference, \(\omega_{input}\) is the disconnect device input shaft speed, \(\omega_{output}\) is the output shaft speed, and \(\epsilon\) is a small threshold. Next, the disconnect device engages when the speed difference is minimal and front motor torque is near zero. Finally, torque is redistributed between the motors:
$$
T_{total} = T_{front} + T_{rear}, \quad \Delta T_{front} = -\Delta T_{rear}
$$
ensuring no abrupt changes in wheel torque. This phased approach mitigates shocks and surges, enhancing the drivability of electric cars.
For 4WD to 2WD transitions, the process starts with torque transfer, where front motor torque is reduced to zero while rear motor torque increases proportionally:
$$
T_{front} \rightarrow 0, \quad T_{rear} \rightarrow T_{total}
$$
This is followed by disconnect device disengagement, aided by small oscillating torques from the front motor to unlock the mechanism. Finally, the front motor decelerates to zero speed and shuts down. The entire sequence ensures seamless mode changes, which is critical for maintaining comfort in electric cars, a priority for China EV brands aiming to compete on quality.
To evaluate drivability, we defined seven evaluation scenarios that cover common driving conditions for electric cars. These include static, creep, constant speed, coasting, braking, standing start acceleration, and overtaking acceleration tests. For each scenario, objective metrics such as acceleration jerk, torque consistency, and vibration levels are measured. Jerk, defined as the rate of change of acceleration, is calculated as:
$$
j = \frac{da}{dt}
$$
where \(a\) is vehicle acceleration. Lower jerk values indicate smoother transitions, which are desirable in electric cars to avoid discomfort. Additionally, torque deviation during mode shifts is quantified as:
$$
\Delta T_{dev} = |T_{actual} – T_{expected}|
$$
where \(T_{actual}\) is the measured wheel torque and \(T_{expected}\) is the ideal torque based on driver input. These metrics provide a comprehensive assessment of drivability, essential for refining China EV models.
We conducted real-world tests on a prototype electric car equipped with the disconnect system. The vehicle parameters are summarized in Table 1, highlighting key specifications relevant to electric cars, such as motor power and battery capacity. These parameters are typical for mid-size China EV sedans, emphasizing the practicality of this research.
| Category | Parameter | Value |
|---|---|---|
| Vehicle Parameters | Length / Width / Height (mm) | 4980 / 1915 / 1490 |
| Wheelbase (mm) | 3000 | |
| Energy Capacity (kWh) | 111 | |
| Front Motor | Peak Power (kW) | 202 |
| Peak Torque (Nm) | 306 | |
| Rear Motor | Peak Power (kW) | 253 |
| Peak Torque (Nm) | 450 |
The drivability evaluation results, shown in Table 2, demonstrate that the control strategy effectively minimizes perceptible shocks and surges across all scenarios. For instance, during static mode transitions, no noticeable jerks were detected, with only occasional audible clicks that do not cause complaints. In acceleration tests, the vehicle maintained linear acceleration feel, confirming the robustness of the system for electric cars. These findings underscore the potential of such technologies to enhance the appeal of China EV products in competitive markets.
| Scenario | Evaluation Result |
|---|---|
| Static 2WD to 4WD | No perceptible shock or surge; occasional metal engagement sound not objectionable. |
| Static 4WD to 2WD | Smooth transition without noticeable disturbances. |
| Creep 2WD to 4WD | Stable vehicle speed; no perceptible shock or surge. |
| Creep 4WD to 2WD | Consistent creep behavior; no drivability issues. |
| Constant Speed 2WD to 4WD | Steady speed maintained; no perceptible shock or surge. |
| Constant Speed 4WD to 2WD | Smooth transition; speed stability ensured. |
| Coasting 2WD to 4WD | Linear deceleration; no perceptible shock or surge. |
| Coasting 4WD to 2WD | Consistent deceleration feel; no disturbances. |
| Braking 2WD to 4WD | Smooth deceleration with linear feel; no perceptible shock or surge. |
| Braking 4WD to 2WD | Stable braking performance; no drivability degradation. |
| Acceleration 2WD to 4WD | Linear acceleration; no perceptible shock or surge. |
In summary, this study on electric cars with disconnect devices demonstrates that optimized control strategies can significantly improve drivability and energy efficiency. By analyzing vehicle configurations, mode transitions, and evaluation metrics, we have developed a framework that enhances driving comfort and performance. The results validate the effectiveness of the proposed approach, contributing to the advancement of electric cars, particularly in the China EV sector, where such innovations are crucial for market leadership. Future work could focus on adaptive control algorithms and broader operational conditions to further refine the drivability of electric cars.